Purpose of pulping
Mechanical and chemimechanical defibration methods
Sulfate pulping processes
Terminology in sulfate pulping
Cooking chemicals
Cooking control and reactions

Purpose of pulping

Purpose of cooking in chemical pulp production is to use chemicals and heat to remove fiber binding lignin that chips defibrate easily. Fibers containing cellulose are tried to keep as long, unbroken and strong as possible. Also wood extractives which can later cause foaming and precipitants in the process are tried to remove. Today sulfate cooking is the most commonly used pulp production method.

Chemicals which dissolve as much lignin and as little cellulose as possible are used in pulping. Sulfate process uses white liquor, a mixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S). Sodium hydroxide degrades lignin and sodium sulfide fastens cooking reactions and decreases cellulose degradation caused by sodium hydroxide. Temperature in sulfate pulping is normally 150 - 170 °C.

Lignin amount left in fibers is expressed with a kappa number . Lignin causes pulp to turn brown during cooking. Because bleaching chemicals are much more expensive than cooking chemicals, as much as possible of the lignin is tried to remove during the cooking process. However, too extensive lignin removal causes cellulose degradation to increase. This decreases pulp strength and yield . Today, typical kappa number for pulp to be bleached is 14 - 20 for hardwood and 25 - 30 for softwood pulp. If the pulp is not beached, the kappa number after cooking will be much higher, typically 40 - 100. Pulp yield is typically 50 - 53% for hardwood and 46 - 49% for softwood .

Controllability and smoothness of the cooking process are requirements for succeeding of the following process phases . Digester plant faults reflect to other departments and cause changes for pulp properties such as strength, brightness and beatability changes, debris and brightness reversion.

Mechanical and chemi-mechanical defibration methods

In mechanical pulp production chemical treatment is not used. Defibration is achieved using mechanical stressing and heating wood material . The energy brought to wood transforms to heat and enables separation of fibers by mechanical work. Combination of chemical and mechanical pulp production is called the chemi-mechanical method, CTMP , where chips are defibrated mechanically after a short chemical treatment.

Sulfate pulping processes

Cooking processes can be divided into two main categories. They are the batch method and the continuous method . During batch process the pulp is cooked phase by phase in each digester . There are several digesters in a digester house. During continuous cooking chips and chemicals are continuously fed from the top and removed from the bottom of the digester. Digester is divided to zones, in which different phases take place.

Both batch and continuous cooking have several variations, which are usually based on various liquid changes during the cooking. The purpose of changing the reaction conditions is to improve pulp quality, to enable cooking to a lower kappa numbers and to decrease energy consumption.

Kappa number after cooking has decreased during last decades in order to minimize environmental load. For example in the 1970's the typical kappa number after cooking was 35 for softwood. Nowadays it's usually below 30 . This decreases lignin amount to be removed during bleaching and consumption of bleaching chemicals. Because the removed lignin goes to bleaching line effluents, it is beneficial for reducing the effluent load to continue cooking as long as possible. However, there is no use to decrease the kappa number too much during the cooking, because the pulp strength properties and yield will decrease considerably after a certain point . Rapid development of oxygen delignification has caused that instead of trying to reach very low cooking kappa numbers, more lignin is removed during the oxygen phase .

Terminology in sulfate pulping

Cooking chemicals

White liquor is a chemical mixture used in sulfate pulping. The effective chemicals of it are sodium hydroxide (NaOH) and sodium sulfide (Na2S). The concentration of those compounds in white liquor is expressed as affecting e.g. active alkali or effective alkali (g/l):

With the following calculator you can examine dependencies of active and effective alkali

Sodium hydroxide and sulfide are expressed in grams per liter of sodium hydroxide or sodium oxide (Na2O) equivalents. Practice is based on sodium contents of the compounds . Conversion factor from Na2O to NaOH is 1.29 and 0.775 in reverse direction .

Sodium sulfide concentration in cooking liquor is expressed as sulfidity (%) . Sulfidity is usually on the level 35 - 45% in modern mills. Reduction (%) shows how completely the nearly inert sodium sulfate has been reduced to useful sodium sulfide. Reduction takes place in recovery boiler. Causticity (%) shows chemical efficiency of white liquor production (causticizing). It shows how much inert sodium carbonate has been transformed to useful sodium hydroxide.

Effective alkali, active alkali and sulfidity are the most important properties of white liquor. Effective alkali indicates OH- ion concentration, active alkali total amount of OH- and HS- -ions and sulfidity HS- and OH- ion ratio.

Alkali charge is usually expressed as percentages of wood. This means alkali amount in relation with completely dry wood.

Due to reaction balance during white liquor production the white liquor concentration is approximately 140 - 170 g/l active alkali as NaOH. White liquor includes also other sodium salts, such as sodium sulfate (Na2SO4) and sodium carbonate (Na2CO3) and small amounts of sulfites and chlorides. All sodium salts can be expressed as total alkali (TTA, titrating alkali, g/l). All sodium compounds are taken into account, such as sodium sulfate and carbonate. Large amounts of sulfate and carbonate in white liquor indicate malfunctions in recovery boiler or recausticizing plant. Because sulfate and carbonate don't significantly participate in cooking processes, they are only unnecessary loads in chemical circulation.The white liquor contains also other substances not reacting in cooking, such as chlorides and calcium compounds. The amount of these so called inert materials depends greatly on mill chemical circulation, for example on white liquor filtration success.

Black liquor is white liquor which has reacted in digester and to which wood compounds have dissolved. Black color comes from lignin compounds colored by alkali and dissolved to liquor.

Green liquor is recovery boiler smelt dissolved to weak white liquor. In other words it is black liquor with organic incinerated. In addition other reactions have taken place, for example sodium sulfate has transformed to sodium sulfide. Green liquor is processed to white liquor in recaustizing plant.

Anthraquinone (AQ) is an organic compound, which has been found to increase cooking yield, especially for lower sulfidity levels. In high sulfidity mills, such as running at 40%, the difference is much lower. High cost of anthraquinone has for now decreased its' use. However, in some cases when a mill's chemical circulation is a bottleneck, its' use has been found to be profitable.

Cooking control and reactions

H-factor indicates relative speed of lignin dissolution. It depends on cooking time and temperature. H-factor's dependency on temperature is very strong due to delignification temperature dependency. Even a difference of couple of degrees in cooking temperature can make a big difference in pulp quality. H-factor has been defined so that 1 hour in 100 °C is equivalent with H-factor 1.

Liquor-to-wood ratio indicates the total liquid amount compared to completely dry wood. It includes all liquids involved in cooking; cooking liquor, possible supplementary liquor and water contained in chips after possible presteaming.

Presteaming means treating chips with steam before cooking. Steam treatment removes air from chips pores and helps cooking chemicals absorption in the beginning of cooking.

Flashing means that hot (130 - 170°C) black liquor from a continuous digester is released from digester to lower pressure. This causes the liquor to boil, which forms steam containing odorous gases.

Carbohydrates are sugar compounds, which form cellulose and hemicelluloses. Delignification means lignin dissolving in cooking liquor. Condensation means lignin reprecipitation from cooking liquid onto fiber surface. Residual alkali indicates amount alkali, which is in cooking liquor after cooking.

VOC compounds (Volatile Organic Compounds) are non-condensable hydrocarbons, which form in chips presteaming, cooking reactions and in all black liquor treatment processes.

TRS compounds (Total Reduced Sulfur) are non-condensable reduced sulfur compounds. In the other words they are VOC compounds containing sulfur. TRS compounds form in cooking reactions and in every black liquor treatment phase. TRS compounds have a strong smell. Even in low concentrations they cause the smell typical for pulp mills. However, odor problems have decreased considerably with modern technology (odorous gas collecting and burning).